Multi-shaft screw extruders of this kind, e.g. those described in literature references (1,2), German Pat. No. 862,668 and German Pat. No. 1,111,154, frequently have sections which act as flow resistances against the axial flow through the machine. Such resistances have been described, for example in (3) and U.S. Pat. No. 3,122,356. They may be equipped, for example, with kneading discs (4), various forms of which have been disclosed e.g. in German Pat. No. 813,154 and German Pat. No. 940,109. Kneading blocks (4) composed of several kneading discs set at various angles have been described in German Pat. No. 813,154 and so called braking screws (5) in German Pat. No. 949,162. These are sections of the screw in which the material is urged to flow in the direction opposite to the spontaneous direction of transport of these sections of screw (6,7). Other flow resistances consist of screw sections in which the throughput of material is in the same direction as the spontaneous transport of this section of the screw but greater than the normal, unrestricted flow (6,7).
Such sections within an extruder, or such elements arranged in it, fulfill widely differing functions, for example, melting, mixing, dispersing, plasticizing, breaking up of dye agglomerates in highly viscous melts, wetting, distribution (3), local supply of large quantities of energy by producing shearing and frictional forces in the material, kneading or damming up for example so as to ensure that the entire cross-section of the screw extruder is completely filled up for the purpose of blocking off various pressure stages, e.g. in evaporator screws. The shaping tool usually provided at the end of an extruder also acts as a flow resistance. All these various flow resistances, which have been mentioned here merely as examples, cause a drop in pressure in the direction of flow through the extruder, and this pressure drop must first be built up by pressure build-up sections situated upstream of the resistances.
It is an object of this invention to optimize, i.e. to find the best possible design from a geometrical and energy exchange point of view, for such pressure build-up zones which must be provided in front of every flow obstruction in multi-shaft screw extruders in which the shafts intermesh and rotate in pairs in the same sense.
Theoretical investigations are known which deal with similar problems in single screw extruders. G. Schenkel (8,9), for example, deals with energy optimized melt extruders, i.e. single screw extruders charged with melt and used for discharging and shaping the melt. For these extruders, G. Schenkel describes in particular the correct choice of the speed of revolution and of the depth of thread. O. Armstroff and H.D. Zettler (3) deal with the present state of the art of optimizing multi-shaft screw extruders which have intermeshing screw shafts rotating in the same sense. To quote: "Such advantageous arrangements of screws can at present be achieved only by trial and error and the actual experience described here provide a valuable but not sufficient basis and guide line for the design of screws. An optimum design is generally obtained only after repeated re-construction which invaviably entail production stoppages so that added to the cost of re-construction are the costs due to production losses." It is also indicated in (3) how the known methods of calculation employed for single screw extruders (10 ) can be applied in a simplified form to the much more complicated configuration of flow found in intermeshing double screw extruders with both screws rotating in the same sense. This method of calculation is restricted to determining the "length of backlog," that is to say the length of product filled section of the screw in which pressure is built up in front of the above-mentioned flow resistances. When the calculated pressure build up lengths are compared with the experimentally determined lengths also reported (3), the correlations are not satisfactory. It is also said in (3) that "the possibility of transferring this method to other production machines still remains to be tested."
German Offenlegungsschrift No. 2,236,902 deals with the problem of designing the pressure build-up zones in front of flow resistances in double screw extruders with intermeshing screws rotating in the same sense with a view to "ensuring low energy dissipation." As means for achieving this, the said German Offenlegungsschrift describes singly thread or double thread "close meshing" trapezoidal screw profiles with a ratio of screw pitch to external diameter of screw of 0.2 to 1.5 for the pressure build-up sections, in contrast to the screw profiles conventionally used in these extruders, which are frequently triple thread, close meshing and scraping, e.g. as described in German Pat. No. 862,668. Apart from the fact that trapezoidal profiles in double screw extruders in which the screws rotate in the same sense are kinematically unable to mesh closely and would, therefore, appear to be less suitable as means of building up a pressure on account of the greater amount of backflow due to leakage, the trapezoidal profiles claimed in the said document are constructed with very wide screw ridges so that the product located there would be subjected to high shearing stress. It therefore appears questionable whether the desired object of exceptionally low energy dissipation could be achieved with the means claimed.